The exponential increase in hip fracture incidence with age cannot be explained either by the age-related decrease in area! bone mineral density (aBMD) or by the increase of fall risk with age. Recent studies have outlined age-related changes in hip structure that appear to result in increased fragility but that may not be reflected in bone density measurements. We propose to examine the effect of aging on proximal femoral structure and strength and to relate these measures to hip fracture risk. To accomplish this end, we will carry out two studies using quantitative computed tomography (QCT): a longitudinal study to examine age-related changes in proximal femoral geometry and strength and a prospective study relating these properties to incident hip fracture. The analytical methods will include structural analysis of the hip from high resolution QCT scans and patient specific finite element modeling (FEM) to estimate hip strength in loading conditions simulating a single-legged stance and a fall to the side. Our longitudinal cohort will be a subset of 500 patients selected from the 5000 elderly subjects in the Age Gene/Environment -Reykjavik (AGES-R) study. In the prospective fracture study the AGES-R fracture registry will be employed to determine which of the subjects scanned at baseline have sustained fracture of the hip. The scans of these subjects will be analyzed along with a set of gender and age-matched controls. In this study, we expect to derive clinically relevant information regarding the effect regarding how the structural characteristics of the proximal femur and its strength change with age and how they contribute to fracture risk.